With the term “exhaust gas” it is intended a flue gas that is produced as a result of the combustion of fuels, such gasoline/petrol, diesel, fuel oil or coal. The ever increasing diffusion of power plants, industrial process plants, and motor vehicles in the world, has urgently led to the study of possible solutions for reducing the harmful effects of the exhaust gases on the environment and on the man.
Indeed, although the largest part of most exhaust gases is relatively harmless nitrogen, water vapor (exception made for pure-carbon fuels), and carbon dioxide (with the exception of hydrogen as fuel), a relatively small part thereof is formed by undesirable toxic substances, such as carbon monoxide, hydrocarbons, nitrogen oxides, partly unburned fuel, and particulate matter.
Generally speaking, with the term “particulate matter” (briefly referred to as “PM”) it is intended solid or liquid particles suspended in a gas. In an exhaust gas, such as the exhaust gas produced by a diesel engine, the main fraction of PM is composed of very small particles, mainly consisting of impure carbon particles (in jargon, also referred to as “soot”). Because of their small size, said particles, when inhaled, may easily penetrate deep into the lungs. The rough surfaces of these particles make it easy for them to bind with other toxins in the environment, thus increasing the hazards of particle inhalation. The discharge amount of PM becomes large in a diesel engine using a gas oil as a fuel or a direct-injection type gasoline engine recently coming into wide use.
A solution for removing (or at least reducing) the PM emissions of an exhaust gas produced by fuel combustion, e.g., in a vehicle engine, provides for the use of a particulate filter. Making reference to the exhaust gas produced by a diesel engine, a particulate filter—in this case, referred to as Diesel Particulate Filter (DPF)—is a device arranged in an exhaust gas emission path of the diesel engine for receiving the exhaust gas and retain the PM included thereinto.
A conventional DPF may consist of a cylindrical body made of porous material, such as silicon carbide (SiC), with a first base (upstream side) receiving the flow of the exhaust gas produced by the engine. Such DPF has a honeycomb structure, with a plurality of exhaust gas flowing channels extending in parallel to the longitudinal direction of the cylindrical body, from the upstream side body to a downstream side, corresponding to a second base of the cylindrical opposite to the first one. These channels are alternatively plugged at either the upstream side or the downstream side to form a checker pattern.
The exhaust gas (including PM) hits the first surface, and is forced to flow through the channels of the DPF that are not plugged at the upstream side. Thanks to the porosity properties of the SiC, the PM included in the exhaust gas is blocked by the walls of said channels, and remain confined in the DPF, while the rest of the exhaust gas (essentially free of PM) crosses the walls, passes into the adjacent channels and exits from the DPF, for being outputted outside the vehicle through exhaust pipes.
Heretofore, as the honeycomb structural body used in the exhaust gas converting apparatus for vehicles, there is well known a one-piece type honeycomb structure (called as a honeycomb monolith) made of a low thermal expansive cordierite. This type of the honeycomb structural body is used by carrying a material having a high specific surface area such as active alumina or the like, a catalyst of a noble metal such as platinum or the like, and an alkali metal for the NOx converting treatment on the wall surface.
As another example of the honeycomb structural body, there is also known an aggregate type honeycomb structural body formed by integrally bonding a plurality of honeycomb structural units (honeycomb segments) comprising a silicon carbide material prepared by extrusion molding.
JP07-054643 discloses an exhaust emission control device manufactured by combining and arranging twelve pieces of filters formed into a honeycomb shape by a porous silicon carbide sintered body. A seal member serving as a heat resistant filling material is interposed between the filters adjacent to each other, and its periphery is covered by a heat insulating member. The seal member is comprised of a ceramic fiber, a silicon carbide powder, and an inorganic binder.
EP 816065 discloses a ceramic structure in which a plurality of the ceramic members are integrally adhered by interposing a sealing member of an elastic material consisting of inorganic fibers, preferably a ceramic fiber, an inorganic binder, preferably a colloidal sol, an organic binder, preferably a polysaccharide, and inorganic particles, preferably inorganic powder or whisker selected from a carbide and a nitride, and mutually bonded three-dimensionally intersected organic fibers and inorganic particles through the inorganic binder and organic binder between the mutual ceramic members. More in particular, the ceramic fiber is selected from silica-alumina, mullite, alumina and silica, the colloidal sol is selected from silica sol and alumina sol, the polysaccharide is selected from polyvinyl alcohol, methyl cellulose, ethyl cellulose and carboxymethyl cellulose, and the inorganic powder or whisker is selected from silicon carbide, silicon nitride and boron nitride. More preferably, the sealing member consists of silica-alumina ceramic fiber, silica sol, carboxymethyl cellulose and silicon carbide powder.
U.S. Pat. No. 6,777,114 discloses that when a silicon carbide-based honeycomb filter having a structure bonded by metallic silicon is used as a DPF and then is reactivated, oxidation reactions under a low oxygen partial pressure may take place causing the destruction of the filter caused by sharp temperature increase due to the oxidation of, in particular, metallic silicon. Further metallic silicon has a property of easily dissolving in an acid when having no oxide film thereon. As a result, when a sintered body containing metallic silicon as a constituent is used as a DPF, the sintered body is exposed to an acidic gas atmosphere generated by the combustion of sulfur, etc. present in the fuel used; and there has been a fear of, for example, the destruction of the filter caused by dissolution of metallic silicon. Therefore, the formation of an oxygen-containing phase at the surface of the silicon carbide particles and/or the metallic silicon or in the vicinity of the surface is suggested to suppress the oxidative decomposition of the silicon carbide and the metallic silicon.
P. Stobbe et al, “SiC as a Substrate for Diesel Particulate Filters”, SAE Technical Paper Series 932495, 1993 discloses that one possible concern of the use of SiC as diesel filter may be that of high temperature corrosion. The authors explain that silicon carbide oxidizes relatively easily in connection with atmospheric air according to reaction (I), thereby forming a tight layer of amorphous silica on the surface of the material.SiC+2O2→SiO2+CO2  (I)
The reference also discloses that at 900° C., the oxidation can reach a depth of about 0.05 μm and this corresponds to a reduction of the contact area (SiC—SiC) between the sintered grains of approximately 0.2%, and at 1500° C., the reduction of the contact area can be about 1%, so that for extreme temperature excursions, the layer can crack and therefore allow local free passage of oxygen to the SiC, and a consequent increase in the rate of oxidation.